The present invention relates to an apparatus for the detection and visual display of the mineral content of osseous tissues, called an osteodensimeter, as well as, according to a more particular utilization, the application of said apparatus to an efficient and accurate process for measuring the demineralization of the spinal column.
It has long been known that the mineralization of the human skeleton increases in parallel to the growth of the size of the person and can continue for some time following the stopping of this growth until the bones reach a maximum weight at about thirty years of age. After this time, ageing causes the reverse phenomenon, the bones are not completely renewed and their weight progressively decreases. The causes of this process have not been completely explained and are in fact numerous, an important part being played by the genetic constitution of the person and the richness in calcium of his diet. Moreover, this demineralization phenomenon, apart from affecting individual persons in an irregular manner, is selective with regards to the actual bones, so that the spinal column, radius and femur are very sensitive to it, whereas other bones remain intact with age. In principle, women are much more subject to this problem than men. Finally, the demineralization can be linear as a function of time, or may become ever faster. The pathological degree of this process constitutes osteoporosis.
The bone affected becomes increasingly porous and finally loses its mechanical characteristics. It may become unable to fulfill its function and this is manifested by the weakening of the trunk of the person, accompanied by spinal pain, spinal deformation or even the rupture of the column femoris. The latter phenomenon causes a 25% mortality and significant invalidity in other cases. Therefore the costs to the public health service are very high and can only increase as a result of the ageing of the population in developed countries. Prevention can be very advantageous, but this requires detection means adapted to the size of the population at risk (30% of menopausal women suffer from osteoporosis).
Known diagnosis processes involve the irradiation of exposed parts of the body. The method widely used for other applications, i.e. x-rays, is inadequate for a precise diagnosis and only provides a general information in the case of already advanced demineralization. The measurements suffer from the difficulty of faithfully calibrating the radiation source and the exposure and processing characteristics of the film.
Gamma radiation measurement methods have already been developed and they have the double advantage of only slightly radiating the exposed tissues and of being collectable by sensors, such as scintillators, able to translate the partial absorption of the radiation. Gadolinium 153 sources have more particularly been used. This material has the advantage of emitting radiation according to two different energy levels behaving differently with respect to bones and soft tissues, so that it is possible to distinguish the energy absorbed by these tissues from that absorbed as a function of the thickness of the bone, which is all that is of interest for the doctor.
This distinction is made in a satisfactory manner by the use of an analyzer integrated into the measuring cascade and consequently there is no longer any need for using the means necessary with radiation sources having a single energy level, such as immersing the part of the body to be examined in a container of water to simulate a constant thickness of soft tissues, which is valid per se, but not very practical when an entire spinal column is being investigated.
The radiation emitted by gadolinium 153 is relatively sensitive to the fat irregularly contained in the traversed tissues, but on attempting to replace this material it is necessary to use a double source. The authors of the invention have examined this problem before and recommend the use of americium 241 (energy 60 keV, half-life 432 years) and barium 133 (356 keV, 10.6 years) as causing few radiological protection problems and therefore being relatively insensitive to the pressure of fat in the tissues (according to "Mesures de la masse minerale osseuse par attenuation de photons gamma dichromatiques" by D. Tola, R. Hours and J. Boutaine, in "Mesure et signification du volume osseux et de la masse osseuse" by Lavel-Jeantet and Caulin, published by Armour-Montagu in 1982). This improved method gives results with an adequate accuracy, whereas the depth reconstitution of the image of the organism by tomography, which constitutes the best existing investigation process, requires costly equipment, completed by a computer and remains a procedure which is too sophisticated for simple osteoporosis measurements.
Supplementary problems which have given rise to the present invention, however, occur in the case of the very important application of this procedure to the examination of the spinal column. It is therefore necessary to stretch out the patient in a supine position with the legs slightly raised, so as to decrease lordosis. Under these conditions, the most advantageous arrangement consists of placing the radioactive source above the patient and the receiver block below the patient, or vice versa. Thus, the rays emitted are vertical and images from the front are obtained, which have the disadvantage of superimposing the image of the spinal column, which is mainly of interest to the doctor, on that of the posterior arc, which is less subject to demineralization. Thus, the accuracy of the findings is prejudiced by the presence of said posterior arc, particularly as its shape is very irregular.
One might be tempted to position the transmitted block, carrying the radioactive source, and the receiver block on the sides of the patient, so as to obtain a profile image, which would isolate the spinal column, but then the difficulty is encountered of satisfactorily collecting radiation on a continuous band of scintillators due to the dimensions of coupled conventional photomultipliers. Thus, the image is formed point by point with the acid of a single sensor facing the source, so that, apart from prolonged immobilizations, it is necessary to provide a double translation movement of the source and the sensor which can be much more easily accomplished in the case of a vertical beam, because the translations then take place in the horizontal plane and only simple mechanical means are required, which would not be the case if it was necessary to provide a vertical reciprocating movement of a heavy assembly.